This GCRC protocol comprises two related phase II studies of monoclonal antibody 81C6 in the treatment of patients with brain tumors. 81C6 is an anti-glioma antibody that has demonstrated cross-reactivity with most breast cancers. The first study examines intrathecal monoclonal antibody 81C6 in the treatment of patients with neoplastic meningitis or postoperative cystic cavity communicating with CSF. Patients with breast cancer are susceptible to leptomeningeal metastasis, and currently available treatments for this condition are inadequate. Major eligibility criteria for this study include: 1) cytological or radiographic evidence of malignancy in the subarachnoid space, and 2) evidence of reactivity of the tumor cells with 81C6. Prior to treatment, peripheral stem cells will be collected and saved. They will be used in the event of hematologic toxicity. For 48 hours prior to the antibody administration, patients will take 2 drops of SSKI twice daily with water or juice. This will continue for 28 days after treatment is administered. The radiolabeled antibody will be delivered as a single dose via an intraventricular or intracystic catheter. At the time of injection, emergency drugs, including epinephrine and benadryl, will be available. Injections will be administered on an in-patient unit, so all other emergency facilities will be available. The dose will be 10 mg of antibody and 60 mCi of radionuclide (131I). Patients < 18 years old will be treated with a dose of 60 mCi a patient m2/1.73. Doses will be repeated at 4 week intervals until hematopoietic toxicity requires infusion of peripheral stem cells or until any other off-study criteria is met. No significant toxicity is anticipated, although bone marrow suppression, liver injury, and evidence of radiation damage to the central nervous system are theoretical possibilities. Three patients (2WM, 1WF) have been enrolled on this protocol since its activation. One patient had radiographic stable disease after evaluation at 8 courses of treatment then developed progressive disease. Two patients had radiographic stable disease after their first evaluation, but developed hemotological toxicity and could not be treated further. All patients experienced grade 3 or greater toxicities of neutropenia and/or decreased platelets and received transfusions. We expect further accrual, and this protocol remains open.